Audio amplifier



March 15, 1960 s. A. CORDERMAN AUDIO AMPLIFIER Filed Feb. 21, 1956 INPUT 2 INVENTOR SIDNEY A. CORDERMAN ATTORNEY United States Patent AUDIO AMPLIFIER Sidney A. Corderman, Binghamton, N.Y., assignor to McIntosh Laboratories, Inc., Binghamton, N.Y., a corporation of Delaware Application February 21, 1956, Serial No. 566,897

12 Claims. (Cl. 330-122) The present invention relates generally to audio output transformers, and more particularly to audio output transformers for class AB or class B push-pull audio amplifiers wherein is required substantially unity coupling between the primary windings of the transformers, to avoid audio distortion during switching of current flow from one to the other tube of the push-pull pair.

The present invention pertains to audio amplifier circuits such as are disclosed in United States Patents No. 2,477,074 and No. 2,646,467, issued to Frank H. Mc- Intosh. In these circuits bifilar winding of transformer primaries is resorted to in class B or AB push-pull output stages in order to assure unity coupling between transformer primary windings at all frequencies in the band to be amplified. The output stage is half plate loaded and half cathode loaded, and the cathode of each tube of the output stage is arranged to remain always at the same A.-C. potential as the anode of the other tube, 'by means of the transformer arrangement.

In accordance with one modification of the present invention, the bifilar winding arrangement heretofore resorted to is dispensed with, and two primary windings are employed which are coupled with considerable leakage. One of these primary windings is connected from cathode to cathode of the output tubes. The remaining winding is connected anode to anode and adjacent ends of the two primary windings are connected, respectively,

to the cathode of one tube and the anode of the other. Provided sufiicient primary inductance is employed the coupling between the windings will be entirely electromagnetic at low frequencies, and will be unity despite the fact that close physical proximity of the winding turns is not present, because leakage reactance is essentially zero at sufficiently low frequency. A condenser is employed to couple the cathode of each tube to the anode of the other tube, the condensers being of relatively low capacity, and functioning therefore to provide unity coupling at the high end of the audio band, where the electromagnetic coupling of the windings is inadequate.

The leakage inductance of the transformer, taken with the coupling capacities, is arranged to resonate at some frequency within the audio band. If the resonant circuit so formed has a high Q, poor coupling will be obtained in the vicinity of resonance. This effect may be eliminated by designing the circuit to have low Q, as by reducing leakage inductance, or increasing coupling capacitance, or both, and thereby decreasing their ratio.

It has been found in practical designs that a value of coupling capacity of .25 p.f., and a leakage inductance from cathode to anode of 25 mf. provides excellent reproduction, while permitting utilization of an inexpensive coupling condenser, and relatively inexpensive transformer construction. Of course, the circuit Q obtained depends not alone on inductance and capacity, but also on the resistance or losses associated with the leakage inductance, and the values of L and C suggested are those which operate well in conjunction with a commercialaudio output transformer of good quality.

2,929,028 Patented Mar. 15,

It is, accordingly, a broad object of the present invention to provide a novel transformer, for use as the output transformer of a class B and class AB audio am plifier, and in which unity coupling is provided over. a wide audio band, by a combination of electromagnetic and capacitive coupling.

It is a more specific object of the present invention to provide an arrangement for unity coupling two windings over a wide band of audio frequencies, in which electromagnetic coupling is relied on at low frequencies and capacitive coupling at high frequencies, and in which the capacitive coupling is provided by a capacitor of high impedance at the low frequencies, and of negligible impedance at the high frequencies. I

Still a further object of the present invention resides. in the provision of a wide band push-pull power amplie fier, in which the amplifier tubes are arranged to pass current in alternation, and in which is employed an output transformer having two primary winding sections connected in series between the cathodes of the tubes, their junction being grounded, and two primarywinding sections connected between the anodes of the tubes, their junction being connected to a source of anode voltage, wherein the transformer primary windings are required to maintain the cathode of each tube at the same A.-C. potential as the anode of the other tube, and wherein. the necessary coupling between the primary windings is provided electromagnetically at low frequencies, and capacitively at high frequencies.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of onespecific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:v

Figure 1 of the drawings illustrates schematically a class B amplifier power output stage, in accordance with the present invention; and

Figure 2 illustrates schematically a single ended modification of the system of Figure 1. I

Referring now more specifically to the accompanying drawings, the reference numeral 1 denotes one tetrode amplifier tube of a push-pull class B amplifier output stage. The reference numeral 2 denotes the remaining amplifier tube of the push-pull pair.

Connected between the cathodes of the tubes 1 and 2 is a center-tapped primary winding 3 of an output transformer 4. The center tap is grounded. A further primary winding 5, having the same number of turns as the winding 3, is inductively related to the winding 3 by reason of a common core. The windings 3 and 5 are not bifilarly related, nor is any extreme attempt made to reduce the leakage inductance between the windings. Rather economy of transformer design and fabrication is an important consideration.

Associated with the primary windings 3 and 5 is one or more secondary windings 6, which may be interleaved with both or either one of the primary windings to provide low leakage reactance.

A condenser 7 couples two adjacent terminals 8 and 9 of windings 3 and 5, and a further condenser 10 couples the remaining adjacent terminals 11 and 12. The terminal pairs 8 and 9, and 11 and 12, are those having substantially identical A.-C. potentials.

The anode of tetrode 1 is connected desirably to terminal 12, and the anode of tetrode 2 to terminal 9. The screen grids of tetrodes 1 and 2 are connected respectively to terminals 9 and 12.

The control grids of tetrodes 1 and 2 are driven from signal supplied at leads 13, 15, respectively, and, are supplied'with negative bias at terminal 14, selected to establish class B or class AB operation, as desired. The driv-' ing signal between 13 and 15 is balanced to ground;

A typical value for coupling condenser 7 and 10 is 2'5mf., "while the leakage inductancebetw'een windings 3 and 5 is of the order of 25 mh.

It has been found that amplifiers arranged in accordance with the above description have low distortion provided adequately large amounts of copper and iron are employed, to provide sufiiciently high low-frequency reactance in the primary windings. It is further found that push-pull amplifiers which operate class B or AB, i.e. in whichthe tubes pass current in alternation are subject to distortion caused by the switch-over from one tube to the other, and that this type of distortion may be eliminated provided the anode of each of the tubes of the push-pull pair is maintained at the same A.C. potential as the cathode of the other tube. To this end resort has been had in the past to bifilarly winding the respective primary windings. The bifilar winding of primary windings of itself introduces problems of cost and of transformer design, because of high interwinding capacity, and because of high voltage gradient existing between adjacent turns of'the respective windings.

.To avoid the necessity for bifilar windings, and to reduce the capacities and voltage gradients inherent in this type of construction, I wind the windings 3 and 5 nonbifilarly, and with no great attention to reduction of leakage inductance.

The windings 3 and 5 are nevertheless unity coupled for low frequencies, because at low frequencies, even considerable leakage inductance does not provide appreciable leakage reactance.

The condensers 7, 10, have low capacity, for the sake of economy, and play no appreciable part in coupling the windings 3 and 5 at low frequencies. However, at high frequencies, where the leakage reactance between the windings becomes of importance, coupling is accomplished readily via the condensers.

There will exist a region, intermediate between the low and high frequencies, for which the condensers 7 and 10 resonate with the leakage inductance between windings 3 and 5, forming a parallel resonant circuit. Poor coupling will exist for frequencies adjacent to resonance if the Q of the circuit is too high. If it is low, however, this elfect will not occur. The Q of an LC circuit depends on the ratio wL/R, w being the resonant frequency in radians. Now a small C allows a high resonant frequency and in turn a high Q. In opposition a large C in turn gives a low Q.

, In accordance with the present invention values of L and C are so selected as to provide a low Q, and hence adequate coupling and no response peaks.

0f course, the actual Q value of the circuit is determined not only by the L-C values of the circuit, but also by theeffective resistance, R, associated with the circuit. This resistance depends on the transformer winding resis'tances and winding operating impedance as determined by the secondary load impedance and primary source impedance in conjunction with the transformer turns ratio. The recommended values of circuit constants, above provided, are suitable for use in high fidelity, push-pull output stages employing conventionally wound transformers, and 'arein any event not critical.

Once an output transformer has been selected for use in a given circuit, a wide range of coupling capacity may be employed, without departing from the principles of the present invention, and if for any value of coupling capacity non-linearity of response is observed for range of a frequency within the audio band, this non-linearity may be eliminated by re-selecting the coupling capacitors so as to further decrease the Q of the circuit which includes the coupling capacitors. When an adequately low Q has been attained the frequency response curve of the amplifier will be 'Hat, and the total leakage reactance between transformer primary windings be zero, or practically zero, over the entire audio band, and in fact for frequenciesfar above this band, Since the reactance of the coupling capacitors continues to decrease as frequency increases.

Values of capacitance C of .25 pf. have have been successfully employed. The secondary windings 6 are preferably coupled very closely to the cathode primary windings, 3, while the plate primary windings may be coupled loosely to the cathode primary windings 3 and to the secondary winding 6. Leakage inductance from the cathode pin to the anode pin, in an actual system, was measured as 25 mh.

Advantages of the present system are that insulation problems are reduced, between primary and secondary windings, since electromagnetic coupling need not be tight. For low impedance loads, such as loudspeakers, the secondary winding operates at or near ground potential. Therefore, the secondary and cathode primary windings'may be interleaved and-little insulation is required between these windings. On the other hand, when the output circuit is used to modulate a plate supply to R.F. tubes in a transmitter, insulation advantages can be obtained by interleaving the secondary with'the plate Winding.

The system may be employed in output transformers which have auxiliary windings, for bringing bias and screen supply voltage to output tubes. Such auxiliary windings'may be coupled electromagnetically at low frequences and capacitively for high frequencies.

In accordance with a modification of the present invention, triodes 20, 21 are driven in push-pull relation at terminals 22, 23, the balance or ground point being 24. The triode 20 is cathode loaded by means of a primary winding 25, connected between the cathode of triode 20 and ground, or B- terminal .26. The triode 21 I is plate loaded by a primary winding 27 connected between B-i-terminal 28 and theanode of triode 21, and has its cathode grounded. A .secondary or output winding 29 is electromagnetically coupled to primary and secondary windings 25 and 27. The windings 25 and 27 are loosely coupled electromagnetically, but are further coupled by a condenser30, connected between the cathode of triode 20 and the anode of triode 21.

It is assumed that the power supply, .connected between points 26 and 28, is low impedance, or of negligible impedance, at all frequencies. The condenser 30, the value of which is selected is in the circuit of Figure 1, provides coupling at high frequencies, and represents a relatively high impedance at low audio frequencies. At low frequencies coupling between .the windings 25, 27 is electromagnetic. Unity coupling is attained over the entire audio band, by means of the recited arrangement, because leakage reactance is essentially zero .at low frequencies even for relatively loosely coupled windings. At higher frequencies unity coupling is provided .by the capacitive coupling. The coupling capacity and leakage reactanceare selected to resonate within the audio band, contrary to normal practice, but a resonance peak does not occur due to the low Q of the circuit.

.In the arrangement of Figure 2 the output voltages at terminals 22, 23 must be selected to compensate for the degeneration in the cathode'circuit of triode 20, i.e. input voltage at terminal 22 must be greater than at terminal 23. The amplifier of Figure 2 may be biased for class A, AB or B operation.

While I have described and illustrated one specific embodiment of the present invention, it will become apparent that variations of the specific details of construction may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

.1. A transformer .for a .band of frequencies covering at leastcight octaves, said transformer including two primary windings, and a secondary winding on a common magnetic core, said primary windings having inherent predetermined leakage impedance, :said primary windings each havingafirstand asecond terminal, a: condenser confleeting the'first terminals of said windings, a condenser connecting the second terminals of said windings, said condensers having a relatively high impedance at the low end of said band of frequencies and a negligible impedance at the high end of said band, and having a capaci-- tance which taken with said leakage impedance resonates far above said low end of said band but far below said high end of said band, said capacitance further selected to provide approximately unity Q resonance with said leakage impedance that the frequency response curve of said transformer is substantially flat over said band of frequencies.

2. In combination, in a push-pull audio amplifier having first andlsecond amplifier devices, said devices having each an input electrode, an output electrode and a common electrode, means biasing said devices for alternate current conduction, and means for driving said'devices in balanced relation, the combination of an output transformer for said devices, said output transformer having a first primary winding connected between the common electrodes of said devices and a second primary winding connected between the output electrodes of said devices, saidofirst and second primary windings being arranged to maintain the output electrode of one of said devices and the common electrode of the other of said devices at approximately the same alternating potential, and for maintaining the output electrode of the other of said devices and the common electrode of said one of said devices at approximately the same alternating potential, said first and second primary windings having substantially unity coupling at low audio frequencies and relatively high leakage impedance at high audio frequencies, said first winding having a first and a second terminal, said second winding having a third and a fourth terminal, said windings arranged to maintain said first and third terminal at approximately equal A.C. potential and said second and fourth terminal at approximately equal A.C. potential, a first capacitor coupling said first and third terminals, a second capacitor coupling said second and fourth terminals, said capacitors having negligible reactance at high audio frequencies and relatively high reactance at low audio frequencies, said capacitances and said leakage reactance having a resonant frequency intermediate said high audio frequencies and said low audio frequencies but far above said low audio frequencies and far below said high audio frequency and a ratio of leakage reactance to total capacitance selected for providing approximately unity Q and thereby reducing the resonant response of said leakage reactance and said capacitances at said resonant frequency to a value which excludes a substantial response peak at said resonant frequency.

3. The combination according to claim 2, wherein the values of capacitors are of the order of 25 microfarads, and the value of said leakage reactance is of the order of 25 millihenries.

4. The combination according to claim 3, wherein the common electrode of one device is connected to said first terminal, and the common electrode of the other device is connected to said second terminal, and wherein the output electrode of said one device is connected to said fourth electrode and the output electrode of said other device is connected to said third electrode.

5. An amplifier, including a first amplifying device having a first signal input electrode, signal collector electrode and common electrode, a second amplifying device having a second signal input electrode, signal collector electrode and common electrode, means for applying signal to said input electrodes in push-pull relation, a first transformer primary winding connected between said first common electrode and a reference point, a second transformer primary winding connected between said second collector electrode and a source of DC. voltage, said second common electrode being connected to said reference point and said first collector electrode, said windings having negligible leakage impedance at low audio audio frequencies, a capacitive circuit including at least one condenser coupling said first common electrode and.

said second collector electrode, said capacitive circuit having negligible reactance at said high audio frequencies and high reactance at said low audio frequencies, wherein said leakage impedance resonates with the capacity of said capacitive circuit at a frequency far above said low audio frequencies but far below said high audio frequencies and wherein the circuit of said leakage impedance and said reactance possesses sufliciently low Q by virtue of the ratio of capacity to leakage inductance of said circuit and the resistance of said circuit that the response of said transformer is fiat for allaudio frequencies.

6. In combination, in a push-pull amplifier having first and second amplifying devices connected in push-pull relation, means for biasing said devices for alternate current conduction in response to a push-pull drive signal, the combination of an output transformer for said devices, said output transformer having two windings, said two windings having unity coupling at the low end of a band of frequencies to be amplified by said amplifier and relatively high leakage impedance at the high end of said band, at least one condenser unity coupling said windings at the high end of said band, the leakage reactance of said windings being selected to resonate with the capacitance of said at least one condenser at a frequency intermediate said high end and said low end of said band but far above said low end of said band and far' below said high end of said band and to have a sufiiciently high value relative to said capacitance to provide a fiat freqency response for said amplifier.

7. The combination according to claim 6 wherein the Q of the circuit comprising said leakage reactance and said at least one condenser is of the order of unity, whereby substantially noresonant rise of response of said transformer occurs at the resonant frequency.

8. In an amplifier, an output transformer having two windings, said windings being on a common core in inductively coupled relation, the electromagnetic coupling of said windings being substantially unity at the low end of a band of frequencies to be amplified and relatively weak at the high end of said band, means including at least one condenser for coupling said windings capacitively with weak coupling at said low end of said band and with substantially unity coupling at the high end of said band, the resonant frequency of the circuit comprising the leakage impedance of said windings and the capacity of said means for coupling said windings capacitively falling intermediate said low and high end, and far above said low end but far below said high end, the Q of said circuit being sufficiently low to assure substantially equal response of said transformer at said low and high ends and at said resonant frequency.

9. In combination, in a push-pull audio amplifier having first and second amplifier tubes, means for biasing said tubes for alternate current conduction, and means for driving said tubes in balanced relation, the combination of an output transformer for said tubes, said output transformer having a first primary winding connected between the cathodes of said tubes and a second primary winding connected between the anodes of said tubes, said first and second primary windings having adjacent terminals, said first and second primary windings wound with substantial leakage impedance, at least one condenser coupling said adjacent terminals of said first and second primary windings, said at least one condenser having high capacitive reactance at low audio frequencies, and negligible capacitance reactance at high audio frequencies, said capacitive reactance and said leakage impedance providing a circuit resonant intermediate said high and low audio frequencies, and far below the high end of said band but far above the low end of said band, said resonant circuit having a low Q, said Q being sufficiently close to unity to provide an essentially flat response over.-

anemone '10. A system for establishing unity coupling over a.

wide band of frequencies by means of a transformer, comprising a first winding, asecond winding, said first and second windings being unity coupled substantially only electromagnetically at only the lower end of said band of frequencies, means including at least one condenser unity coupling said first and second windings only capacitatively at only the upper end of said band of frequencies, said transformer having a Qsuificiently close to unity to maintain said unity coupling at all frequencies intermediate said low end and said upper end of said band of frequencies, and having a resonant frequnecy intermediate said lower end and said higher end of said band of frequencies, and far above said lower end but far below said higher end, said transformer having a sufficiently low ratio of leakage impedance to the capacitive reactance of said capacitance of said means unity coupling said first and second windings primarily capacitively to provide said 'Q. 7

- 11. A coupling device comprising, a first winding, 2. second winding, a common core for said first and second windings, said windings and said core designed to provide substantially unity electromagnetic coupling at frequencies below 100 cycles per second, and to have substantial leakage reactance at frequencies adjacent 10,000 cycles per second, means for capacitively coupling said windings, said means for capacitively coupling including at least one condenser, said condenser having high reactance at 50 cycles per second and negligible reactance at 10,000 cycles per second, said condenser selected to form with said leakage impedance a parallel resonant circuit having a Q of the order of l, and a resonant frequency of the order of 2-7 kc./S.

12.111 combination a push-pullaudio frequency amplifie'r, including a first output vacuum tube and a second output vacuum tube,each of said vacuum tubes having a control grid, an anode and a cathode, means biasing said vacuum tubes for alternate conduction, an output transformenha'ving one primary winding connected be-.

tween said cathodes and another primary winding'con nected between'said anodes, means for maintaining substantially unity coupling of said primary windings over at least the entire audio frequency band, comprising means for establishingsubstantially solely unity electromagnetic coupling between said windings at the lowest frequency of said audio band and substantial leakage impedance at the higher frequencies, means including at least one condenser for establishing substantially unity capacitive coupling between said windings at the high" frequencies of said band and low capacitive coupling at the .lower'frequencies, whereby leakage inductive imped tain substantially unity coupling between said windings for frequencies adjacent those for which resonant responses occur.

References Cited in the file of this patent- UNITED STATES PATENTS 2,646,467 McIntosh July 21, 1953 FOREIGN PATENTS France Jan. 17, 1944 Australia July 30, 1951 

